Update (from an earlier post): the last time I went up to the mountains to snowboard the TPMS light came on after my 4R was parked for 7hrs. After about 30mins of driving, it went away. I initally thought it was the 4 tires and not the spare because I usually keep the spare @ 40+psi. It's been awhile since I've last check the spare (approx 10 months or so) it must've lost a decent amount of pressure to trigger the light when it got cold.
Anyways, what I find interesting is that about a month ago, I started hearing a clunk/rattle coming from the rear of my 4R whenever I went over speed bumps or make sharp turns. I thought it might've been from either the snowboard carrier or my roof cargo rack, but sure enough, it was the spare tire shifting around. It lost enough pressure that it was no longer secured snugly and had enough play to swing around a wee bit. Interesting enough though is that my TPMS light hasn't come on at all since that skiing trip. I haven't had any time lately to bust out my 15gal air compressor, but I did tighten the spare tire carrier cable a little bit more. Just curious if anyone else had their spare tire come a bit loose due to low pressure.