My memory is a bit fuzzy, but if I remember correctly Luke was supposed to go to the rendezvous point with R2-D2 after they were attacked on Hoth. Luke tells the other rebels he will meet them at the rendezvous point as well.
However once he gets in his ship he changes course to go to Dagobah where (if I understood the story correctly) he spends months, with no communication with the rest of the rebels, training with Yoda.
Did anybody know he had gone to Dagobah? Did anybody even know he was missing? Why didn't the rebels go looking for him?