I guess if is deliberate, it might be interesting to look who would benefit from it? It hurts American diplomats but it also hurts Cuba's position. Would stand to benefit from Cuba looking bad or from hurting the improved relationship between Cuba and US.
There was talk of using something like a passive listening device, say something embedded in the walls or furniture that's not powered but has to be activated with a ultrasound beam. Something like The Thing (https://en.wikipedia.org/wiki/The_Thing_(listening_device) but miniaturized?