Absorption or compressor type?
Either way, "money for old rope" is the phrase that comes to mind.
I would always pull them out (or dismount them, if necessary) and give them a good blowing out (or vacuuming, if you insist). If floor mounted, you may as well clean underneath (that is, the floor as well) - no-one else will.
Record the temperature inside each unit. Should be 2 °C IIRC. Take a look at the door seal, and record its condition.
If it was me, I would finish off with a nice blast of Mr.Sheen
(followed by a minute's worth of elbow grease). Not forgetting the electrical safety test, of course.
As always, documentation is "the thing" - and the EST and temperature records "prove" (kind of) that you actually did the PM. If you can't find a serial number, or can't decide which one to record, stick one of your own somewhere amongst the gubbins
around the back.