2015 XF - symptoms at this point:
press "start" with no foot on brake - all systems expected to function come online (HVAC and vents, infotainment center, instrument panel/information center with expect message to press brake to start)
push brake pedal with some force and press start button - instrument panel goes dark except gear selection which shows in park, gear selector know shows in park but not popping up

diagnostics / steps so far - limited as car is not at home
1. hooked up jumper cables to main battery and let run for a while, repeat steps above, same results; note "jumping" vehicle shows no indicator of additional electrical load while hooked up or while attempting to start the XF
2. went through all fuse panels for any fuses that might be related to brake pedal sensor and various instrument, ignition, control modules - all good (wasn't pleased with the inaccuracy of the manual vs the fuses - some were not the indicated spec and some were not where the manual said they should be and I am the original owner and the car has never had an electrical issue warranting fuse replacement).

Headed out now to run through the following:
1. perform voltage check on the battery - if low, remove and take to local auto parts store to check the battery, its about 3 years old
2. pull ODBC codes - not expecting any as no indicators of codes in the dash panels, but checking anyway
3. double check emergency brake is set - attempt to set/unset see results in case this is relevant
4. attempt to manually rotate gear selector - not sure if this is supported in this generation XF, but just in case some "fiddling with it" produces some change in results
5. stand REALLY HARD on the brake while attempting to start
6. attempt to disengage the drive train / get into neutral with the underhood mechanism and see if this will allow it to start
7. come back - report results here.

Full disclosure - the car has been without the "secondary battery" ever since I replaced the main battery about 3 years ago - save me a good chunk of change and kept the start/stop system off which was a huge + for us; has never had an issue other that telling us at startup that it was missing.

Interested in any thoughts on this. Apologies up front if there is already a thread on this somewhere - spent a couple hours searching the forum and didn't see anything the "perfectly lined up" with these symptoms. Won't mind being ridiculed if anyone refers me to existing threads for this generation XF and insults my inept search efforts!

 

Some good news - got it to start and got it home in my garage. What I did...
1. checked voltage - read 12.67 V - maybe enough, but perhaps a surface voltage reading not sufficient for the XF's sensors to want engage the start sequence.
2. Hooked up jumper cables to my diesel pickup truck (big alternator) and checked voltage on the XF battery - 13+ volts
3. with truck running, got in the XF, checked parking brake - no indicator it was set, so set/unset/set
4. stood VERY HARD on the brake (no longer had any power brake system assist with all the attempts previously to start) so really pushed hard to ensure the XF registered I was pushing the brake down
5. attempted to start and it fired right up just like new

What's next since I now have it home but not certain the root cause...
1. its in the garage with my charger hooked up - will let it run its diagnostic/repair cycle until it registers either full charge or bad battery
2. the car is rarely driven - its my wife's and she sometimes drives it a few times a week and sometimes it sits for 2 weeks before firing up. When she does drive it, most of the time its for 3 - 5 miles, shut it off, do something, start, return trip, shut it off. So if alternator is going or battery is aged to the point its not charging well, entirely plausible the root was an insufficiently charged battery. So depending on what my charger tells me, I may put a new battery in it but I will also see what the charging voltage is right after a cold start. Not likely a great test of charging system, but I'll at least have a data point.

Welcome anyone's opinions / suggestions on where I go from here.

 

A last follow-up... its been two weeks since the original "Jag won't start" event. Everything continues to point to week battery kept it from starting, followed by continued "attempt to start" events resulted in loss of pressure to brakes which make brakes hard to push which meant "foot on brake pedal" sensor during starting was not triggering after a while, which explains why when I attempted to jump, it still would not start. Once I did the research online, hard push on brakes with jumper cables connecting jag to my truck - car started. Gave the battery a good charge and ever since it has been fine. The battery is about 3.25 years old and we live in a warm dry climate with hot temperatures in the summer - especially in our garage. Likely reduces battery life when just sitting. All this said, wife has driven the car numerous times since the original "event" 2+ weeks ago and no repeats... and, we've learned a thing or two

 

Make sure that your brake lights are coming on when pushing on the brake pedal.
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From your last post it sounds like the car is just not driven enough to keep the battery charged. Check alternator output and if it's around 14.5V+/- then it's OK. Connect to the jumper points under the hood while engine is running is the easiest way. 12.67 volts after the battery has rested about 15 minutes since last electrical usage (important) is a fully charged battery. Electrical usage includes anything as minor as opening/closing door.