Ok, you found them, now you'll mail them by regular mail and won't have a proof that you did. Most gringos send it by regular mail - I mean most of those very few that do bother to mail it, in the first place.
Yes, officers enter it in computer when issue. No, I have never heard of anybody having issues with cards not returned. Neither have I heard of anybody being denied a new card after a few months because he hasn't returned the previous one. Just make sure you don't leave the old one in your passport when handing it to Migra asking for a new card :)... One guy did, and was yelled at, and then was issued a new card.
I haven't heard of anybody having any issues with any abuse of their visitor status, other than when openly committing a serious crime. Tourists don't return their cards, they don't bring their vehicles back if TIP is not required, they bring several times more stuff than $75 value allowed per tourist ($300 if flying in) and then they leave this stuff in Mexico because they bought a home there - a tourist, yes. At some point the authorities will start enforcing some of this - maybe.
This is the first time I hear of somebody asked why they didn't stamp "salida" in passport when leaving Mexico.
Once every few years somebody has minor problems when they travel all over the country WITHOUT any tourist card (which is only allowed within 30 miles from the border). They are fined $100 or 200 and set free.
