It's safe to drive unless the leak gets worse. If coolant drops and the engine overheats its toast. And that could happen in 30s if a pipe splits. Replacement engine typically >> value of the car...
If it were me I would not drive it until I tracked down the leak...

 

Oh okay. Thank you!

 

No worries.

 

Oh okay. I didn't realize it could happen so fast. I will definitely get this checked sooner than later. Also, how would I check if it's a float issue or not? I tried opening the cap and seeing inside but I cannot see anything in there that resembles what has been written about it in some other posts. I also looked on youtube if I could find a video that shows this but I couldn't find anything, unless I wasn't checking for the right thing. Would it be possible to know where to get more documentation or details on this?

Thank you!

 

If the coolant level is dropping it's not a float issue. The coolant is going somewhere - ie leaking. It could be the expansion tank itself (it can leak), the cap, or any of dozens of plastic pipes and joints
It's also possible it's got more than one leak.

Any decent garage can leak test it. Or if you look around then more than likely you can find where it is. Although could be buried way down somewhere...

Good luck tracking it down. When you find where it is, post on here and someone can advise on the part and how to replace

 

Sounds to me like you have two separate issues: a faulty coolant level sensor and something going on with your gearbox.

As for the RPM running higher, the only thing that can cause that while the car is in drive is the gearbox - if I'm not mistaken, you have an 8 speed ZF gearbox. There is a speed sensor attached to the top side of the gearbox which receives information from the speed sensors on the ABS which tells the car how fast it is going, then the gearbox ECU also receives signals regarding engine RPM which let's the gearbox decide which gear to be in based on its programming (in laymans terms).

Next time you drive on the highway and you notice the high RPM, tap the right side paddle behind the steering wheel to put it in manual mode then it will display which gear you're in - my guess is, it will be 7th gear and not 8th gear. My XFR has a 6 speed auto and at 70mph it hovers around the 1200 rpm mark in sixth gear so it sounds about right that you noticed this difference.

As for the low coolant issue, I would recommend replacing both the reservoir as well as the sensor because chances are that reservoir is cracked and leaking down the side (had this issue with my old 3L V6) so you might as well just replace the whole thing. The plastic gets brittle and cracks and the pressure in the cooling system drives the coolant out the crack - you can see this just with the car idling actually.