The arabic graffiti was found daubed and covering fuel tanks of four easyJet planes at airports which cannot be named for security reasons.
Airline security expert Philip Baum said: “Graffiti in itself won’t hurt anybody.
“But the ability of anyone to place a prohibited item near fuel tanks is a concern, of course.”
The probe comes two weeks after 130 people were killed by twisted Islamic State terrorists in co-ordinated attacks across Paris.
Airports in the capital have tightened security since the horrific shootings and bombings amid fears of follow-up attacks.