In fact, do we have any recorded evidence of spacecraft at Mars suffering from atmopsheric decay over time? It's well known that we commonly use aerobraking (not be confused with aerocapture, which has never been attempted):
to reduce our spacecraft's apoapsis' over a period of many months. In fact, MGS was the first spacecraft to ever attempt Martian aerobraking, which resulted in it damaging one of its solar arrays, which got bent back by the forces generated. This aerobraking campaign took place at approximately 120km above the Martian surface.
What this indicates is that atmospheric decay is present around Mars, so it should therefore be expected that any spacecraft in similar low orbits will eventually suffer from orbital decay and reenter, but I can find no evidence that this has ever occurred.
Additionally and specifically relating to MGS, I can find no accurate, authorative source of MGS' final orbit. This NASA JPL factsheet indicates it was in a circular orbit for sometime:
When Global Surveyor arrived at Mars in September 1997, it initially entered a looping, elliptical orbit around the planet that has been gradually circularized through aerobraking. Its winged solar panels — which feature a Kapton flap at the tip of each wing for added drag — supply most of the surface area that slowed the spacecraft by a total of more than 1,200 meters per second (about 2,700 miles per hour) during the entire aerobraking phase. Since the start of aerobraking, Surveyor's orbit around Mars has shrunk from an initial elliptical orbit of 45 hours to the now nearly circular orbit taking less than two hours to complete.
Wikipedia also states:
Finally, from November 1998 to March 1999, aerobraking continued and shrank the high point of the orbit down to 450 km (280 mi). At this altitude, Surveyor circled Mars once every two hours. […] The spacecraft circled Mars once every 117.65 minutes at an average altitude of 378 km (235 mi).
Obviously, end of mission occurred in November 2006. It has been nearly 10 years since then.
So, my question remains, given what we know about MGS' orbit, has it decayed, or is it in the process of decaying? If it is the latter, are there any models on when the decay might occur?
