Runner Bean white patches on leaves

I planted out my runner beans this weekend just gone. I had hardened them off for a couple of weeks, and there's been no overnight frosts but the biggest leaves on a couple of them (plus a couple of my dwarf french beans) have got big white blotches like you get when they get frost nipped. Very strange. The shoots and young leaves seem fine, I guess I'll just need to keep an eye on them.
I've still got the other few I didnt plant out as replacements if needed, but i'm sure I've never had this problem before with runners out at the end of May and no frost...