I have seen these in the past in a subset of animals with AAV9-syn-GCaMP6f. Once they started, they generally grew in intensity and duration over the following days/weeks. The general consensus I got from talking to others was that they were pathological, but like yours they seemed to occur at much slower timescales than I've seen in the literature for waves generated in both epileptiform mice (https://www.nature.com/articles/s41598-018-19925-7
) and non-epileptiform mice (https://www.eneuro.org/content/4/5/ENEURO.0207-17.2017
, also transgenic in this paper).
We saw a lot more of these mice early on when we used higher titers of virus (1e13) and a lot less when we dialed it back to 2-5e12, so it could be viral related, but we never definitely figured out the root cause.