When the temperature is warm enough the bees will go on cleansing flights. The bees wont defecate in the hive so these cleansing flights are important.
Also at this time of year there are hazelnuts (filberts), kitty willows, crocus and a few other things to forage on. Late winter and early spring is heavy with pollen producers so the bees can gather the needed stores to begin brood production and population expansion. So when the weather breaks the bees are doing more than just cleansing flights. If you watch you hive entrance for a while when the bees are flying, even in 40 degree weather you should see some pollen being brought back to the hive.