You know, it looks to me like bubbles formed between the filter and the sensor – like it had expanded then contracted (filter/film). In computers we call this flash freeze caused by using canned air too close to a microchip. Just seems logical that it would be between the two not necessarily the sensor or filter itself.
Edit: Or high preasure air got pushed between them.
On this forum, I have personally been cautioned against telling people to use procedures that could harm their cameras.
Freezing is a very real problem with canned air - I realize that.
CAUTION: I do not advise using canned air - I use it but I don't recommend it for anyone else.