My guess would be that if the a/c had been going full tilt and someone had only recently been in the room who had a shedding stage of coronavirus and had been coughing and not masked, you'd have been in more danger of breathing in virus particles (that had been harbored by and spread by the air-conditioning unit) than you would have ever been in from surfaces.
It seems like you don't have a lot to be concerned about, though, given all that you have described. I know it's hard not to be nervous, but you weren't in close contact with anyone who would be coughing out the virus or spreading it by talking loudly or singing, or anything like that.
Nobody can be 100% certain about such things, but the surfaces issue (and even the issue of droplets still hanging around in the air for awhile after someone has left a closed room) are at this point theoretical only. Contact tracing hasn't yet found, to my knowledge, anyone who got it solely in this way, they all had some extended contact with someone breathing on them. So while nobody can tell you for sure because there is virus to be found on surfaces and in the air (and the water in the sewers, for that matter), whether or not it is enough of an exposure to infect someone is a whole other question and so far hasn't proven to be the problem, the problem is people breathing on one another in close proximity not wearing a mask for an extended period of time. Whatever the case turns out to be, anxiety is never useful. Peace.
I’m a believer in droplet and aerosol transmission more than surface contact. I am wary of doorhandles and hate it when I see people pull down their masks to sneeze on the groceries. Washing hands after being out in public and not touching face seem like reasonable precautions.
I read that colds are caused by common corona viruses, so while one might differ slightly from the next, precautions for avoiding transmission is likely the same for all.
I am wondering if flu season will be much of an issue this year with everyone masked up. I remember a chart from early this year showing flu transmission decreasing as people started Covid precautions