On warm late winter days when  there are
no flowers yielding pollen the foragers do crazy things.
They're attracted to sawdust, woodchips,  cracked corn
(maize), and other grains on which there is floury dust.
Regarding compost heaps, last year there were hundreds of
bees around a friend's compost heap which contained paper
coffee filters with spent, finely ground coffee.  The bees
were gathering it as though it were pollen.  I was once
shocked to see honeybees on wet dog feces in my kennel in
late winter.  Fortunately this was a short term phenomenon
that only one or two bees were engaging in.  As someone else
mentioned, the bees are after water which they sometimes
find on wet leaves, etc. on a compost heap.
 
Bill Morrison