In the real world we have a nuclear hazard trefoil and a biohazard trefoil. Apparently in Charles Stross' Laundry Files universe there is a thaumaturgy hazard trefoil.
Quoting from The Concrete Jungle:
I reach in and carefully pick up a silver locket on a chain; there’s a yellow-and-black thaumaturgy hazard trefoil on a label dangling from it, and NO PULL ribbons attached to the clasp
Are there any images depicting it about? From the book cover, or maybe from a tie-in game, or even some fan art that the author has said "looks about right."