Can subcutaneous hematomas cause little to no pain?

Of course, but: It depends on the location and size of the hematoma. Hematoma refers to an actual collection of blood rather than a bruise, which is more like a soaked sponge, with tissues being the sponge. Both will eventually reabsorb on their own, but hematomas may do better and resolve faster if aspirated with a needle by an expert in surgical anatomy.

It can happen..: Often, small bleeding/hematoma or bruising, especially in elderly on aspirin/blood thinners, do not cause any significant pain/discomfort. Many don't even know how they got the bruises etc.. The skin itself is sensitive, but deep under skin there is less nerve endings and thus trauma/bleeding in deep area may not cause much pain at all, unless large and pushing on other tissues. Consult doc..