Agree, it generally means they're in a vehicle. You may also hear some say "portable" which would indicate they're on their HT. It's a bit more rare, but you could also even hear "aeronautical mobile" (plane) or "marine mobile" (boat).
The rules used to state that if you weren't at the location of your license, you had to indicate that using the sufixes you're hearing, but they also had to add the FCC district they were located in. For example, if I was driving around near my home, I would need to ID as "N5IMS Mobile 5" (it was generally allowed if the district you were in matched your license's district you could omit that part so the ID would be "N5IMS Mobile". If I was in California (FCC district 6), my ID while driving would then be "N5IMS Mobile 6" or if I was at a temporary base station location it may be "N5IMS Portable 6".
If I moved to California, the FCC would issue me a new call sign for district 6 and my N5IMS call would no longer be valid. They stopped forcing call signs to be aligned with the FCC district several years ago and these rules were dropped.