A LOT of people actually do this...It happens every night in my area as well...and I'd guess, every area out there...
Think about it this way, if you are a 20-60 year old guy with no kids/family, this would probably be my preferred way to take gyms/get coins too since there is less chance for turnover (note it's LESS, not impossible).
One would have to think the cost of gas is more than the 10 coins, BUT...there's that stardust as well so financially, you're probably better off just spending money. Of course, if you have a cluster of 5 gyms real close by, maybe it's worth it, but prob not.
I'd guess they are either in a car that you can't see or possibly behind a building, in a building, etc...
Also, is this happening on just 1 gym or all 5?
That's more likely than a spoofer IMO.