For the second mission in a row I have observed the air raid siren going off, hear the sound of bombs falling and my airfirld hit as I am on my outbound leg after having taken off. The problem is is that there are no aircraft attacking. Below is the basic info and I will fly another mission soon to see if I encounter it again.

Assignment:
Unit - 1 RFC in Flanders
Airfield - BalleuTown
Mission Dates - 26 & 27 May, 1915

Bombs only come from aircraft, we can't drop them any other way so they aren't ghosts if they are dropped bombs the aircraft are (or were) there somewhere - maybe very high. Clouds block labels (and range of labels depends on your label type anyway and settings in Workshop). Change them to Activity will make the range a lot longer at the risk of spoiling immersion.
OR your airfield is near the front line and the advancing enemy is shelling you !

Thank you for the feedback, I will investigate further. Follow on question out of curiousity becasue I don't know. If there are enemy aircraft present won't the TAC indicate their location? I run with the TAC on All and will try being selective with it. Also to add a bit more in both instances it appeared that only one bomb hit and both times in the middle of the airfield. We are close to the front, about 11km so we would be a target for a bombing run.

Yes they will be on TAC if set to ALL or Aircraft, and in "TAC range" which can be changed - otherwise no. Also, I said above also they will not show if hidden by cloud between you and them. Of course artillery can hit you with a shell if near the very close to the front. There are no means for us to drop a bomb by magic - they all come directly from an aircraft, or a shell from an actual gun.