Clever idea! I hope the IDF reads this thread.
...”where a flammable gas is used”...
Fuel air explosives and other thermobaric weapons pack the biggest punch per pound, and are particularly well suited for tunnels - the suck the air out, and their blast waves are contained for greater effect. They should be able to rig a sprayer/detonator on a jerry can, to drop off a little inside the entrance and come back through after the boom. Send the robot with it.
A raid on HQ with multiple fuel air explosions would set them scurrying in circles for a while.
Aerosol marker agents blown through the tunnels could help identify more, or they could hit a jackpot with human sources or discovering some plans or maps, if they continued operating there, rather than standing outside popping artillery.