Bats have very good homing instincts and remarkably strong route-memory, but they are also inquisitive, which is why you might find isolated ones in sheds or the house. They could be seeking shelter, or they might have chased an insect there.
Entering the living area of a house is probably rare, but they do like undisturbed lofts as roosts, preferably clean ones so in newer buildings free from masses of cobwebs and dust. The modern home insulation might also make the roost temperature more attractive to the animals, particularly for their nursery and hibernation needs – some species at least can be quite fussy, it seems!
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The matter of greatly reduced insect populations is a serious one, affecting very many ecological areas, not just bats and other insectivores. The aerodynamics of modern cars very likely does save countless invertebrate lives by the slipstream lifting them clear; but there are significantly far fewer of the tiny animals anyway. They were not only squashed on the windscreen, but many others died on the radiator.
Those 1950s cars. some with vaguely-American styling, were not very aerodynamically-designed, but since their top speeds in normal, pre-motorway driving were probably below 70mph, that might not have made little difference to their efficiency in days when that seems to have been of lesser significance than now.
Gimmicks like projecting sun-screens above the windscreen, and rear windows sloping the wrong way, would have played havoc with any attempts at air-smoothing. Even without those though, the styling just made them good at slaughtering moths and gnats.
So the modern car kills few insects… but has far fewer to kill. Bad for everyone.