I had this and could not find the problem on my 156...I could not see a crack or break in the lense where moisture would get in...
But it got worse and then found a hairline crack in the lense at the top..

.but no stone chip mark...so not sure what did this...and last weekend..I checked the oil and stupidly slammed the bonnet too hard..only for a piece of the lense to fall out
Chris