OP – are they honey bees or bumble bees?
If they’re honey bees it could be a nearby hive swarmed (when a hive decides to split, half stay and half fly away to find somewhere else but most die eventually) and you got some that didn’t make it.
If they’re bumbleys then I don’t know TBH. Do they look like they are walking around in the soil looking for moisture? You could try putting some water out for them (see below, but use a bowl or something similar). Or as mentioned, badgers.
As for bees drinking, most of the time they get enough water from nectar, but in this weather they do drink. I use a birdbath filled with gravel and enough water so they can stand on the pebbles to drink but also not drown.
As for CCD (colony collapse disorder)? I think its 70% weather, 30% a mix of dozens of factors (a large percentage of that a lack of genetic health / variation IMO). Of all the hives I’ve lost over the last few years its been down to the weather – last spring the queens were put off mating by the weather (so laid only male bees, who don’t actually do anything) and this year the sudden cold spell end of march / early april was just awful 🙁