I think I might be onto something here … just yesterday I was wondering about claims of an image purported to be a “legionary banner” found in that cave full of Christian symbols. If you read my coverage already, you know that I was having great difficulties seeing a banner in what was presented (apparently) as such. Amicus noster Joseph Lauer sent along a pile of links to photos from the site, including a better one of the item in question:
I’ve been staring at this for roughly an hour now, trying my darnedest to see a Roman vexillum or something vaguely military in it. I fiddled with it in photoshop to see if something was ‘hiding’. All to no avail. Then I realized I had seen this thing before … at Dura Europos. Ecce:
It’s that fresco usually dubbed ‘Ezekiel’s vision’. The thing on the right is the Mount of Olives being rent in two with all the resurrected folk spilling out. It seems to me that the incised image from the cave near Jericho is a stylized version of this … a single olive tree on top (the triangular thing) to designate the ‘olive’, and the mountain is split in two down the middle and across. Feel free to comment …
rogueclassicism 
It could be a vexillum, or it couldn’t. I can’t say. We would need a larger image with better resolution, which shows the complete carving, and the surrounding imagery etc. would also be helpful. But even then there would be no guarantee.
On this image — http://wordpress.haifa.ac.il/wp-content/uploads/2009/06/oey.jpg — it looks a lot more like a vexillum. But why isn’t there any heraldic animal carved as well, plus the number of the legion etc.? Maybe because its form was determinative enough? Or they added heraldic paintings? (Just thinking out loud here.)
Admittedly it has the basic ‘shape’ of a vexillum, but beyond that, it doesn’t quite make sense … the triangle at the top (presumably the cords which attached it to the standard) is upside down. There also is the problem of it having two ‘tiers’, which doesn’t quite work. I’m not sure why it would be divided into sectors either if it was a vexillum (that was basically my own ‘thinking out loud’).