In the Nutloose post in
The Kerch Bridge Thread one can see the bent guardrail supports along the edge nearest to the still-standing span essentially pointing away from what I expect was the center of the blast. They also point to about where the fallen span got creased.
Those supports would have been protected from that damage if the bomb was below the bridge deck.
Recall that Mathias Rust got to Moscow in a small plane - never underestimate the inability of a security force to imagine something happening. Look at what was lost when a few guys with box cutters got past airport security because the FAA failed to understand that a hijacker could be an educated pilot and not require the flight crew to remain alive. Security is better after the fact at looking at cameras and records later to unravel the problem.