TPMS question

Here's the deal...when I lifted my truck and put new wheels and tires on it, I didn't bother even trying to get TPMS from my steel wheels to go in the aluminum wheels because I don't see the need for the system at all. SO...there are NO TPMS sensors on my truck at all.
Here's the question...my pressure indicator light on the dash only occasionally comes on, like maybe once in 5 or 6 drives. Anybody know why that might be?? I found it very odd, but there may be a simple explanation.

 

Are the old wheels still in your garage or close to where you park ?

 

LOL...yes, they are. About 20ft away. So, I gather that would make the light stay off for a little while after I leave the house. About how long does it take the system realize the sensors are gone??

 

^20 minutes, per federal regulation.