I'd agree with your guess----seems like someone else brought it back. the one i bought had the filters(for fumes) in sealed plastic bags and when i buy new ones they are in bags. IMHO either take the whole thing back or buy new filters----why take the risk?
my friend who is painter says his, on the job test for the filters is when he can smell the fumes or hold it over a bottle of ammonia/bleach and see if you can smell it.
i believe i read somewhere the fiilters need to be replaced at least every 6 months, after unsealing the package.