We've searched his office for surveillance traps. Residences. Club. Nothing. Or nothing remains.

Those are all possibilities.

Believe the business with his sister was blocking an inappropriate match she intended. (Gather it was a permanent block.) So it could be revenge served cold.

The assistant's a Weasley. Has always seemed entirely sycophantic. Understand Weasley aimed to earn Our Lord's Mark. Gather L thought it unlikely. May or may not have made that plain to W, but I never saw any sign W was anything but starstruck by L. Beside himself with his good fortune at having the position.