They look more like Hazel Nuts to me. Acorns do not have a hard outer shell covering the entire body, its just the cup.
A few years back I bought a ride on tractor mower at auction and threw a tarp over it until I had the time to do some repairs. When I got back to it I found the engine making some rather odd noises when I tried to start it. The space between the engine cover and air intake fan above the crankshaft had been packed with hazel nuts by some enterprising rodent. Possibly mice but more likely the then huge local population of squirrels.
In your case though I wouldn't discount birds either... or, noting the copper pipe off-cut maybe a hungry plumber?
I was clearing a room recently and when assorted junk was removed from one corner there were clear signs of mice having chewed all manner of things including some aluminium foil that had been reduced to tiny pieces! Curiously, there were many walnut shells, which in itself was not surprising as we have two walnut trees close to the the house, but I was a bit surprised that mice could collect such large items and although we have abundant squirrels (which are busy at this time of year burying walnuts in the grass!) I'd be surprised if they were bringing them indoors because I'd expect a lot more signs of damage.
Walnuts! Not a prayer from our lone example, mighty though it is. I'd estimate that 90+% has been squirrelled away by July every darned year. Perishing varmints.