I went into the shop this morning to start a day of work. I opened my plane cabinet and noticed a shoulder plane with rust on it. When I looked closer it was covered in rust despite me always oiling them (or thinking I do!) when I put them away. Then looking at all the other planes they too had a thin layer of rust albeit a little less severe on them. So out came the sandpaper, WD40 and Boeshield T9. Two hours later they were back to normal.
It appears that over the last few days humidity and external temperatures had deviated quite a lot in England. I remember going into the shop at the start of the week, opening the plane cabinet and seeing my breath inside. The shop heating was on but wasn't at full temperature. I thought nothing of it at the time and all tools were fine. The plane cabinet is fairly airtight but me opening the cabinet let in the moist air from the shop and the resulting formation of rust over the next couple of days ate at the tools while I was out of the shop.
I have some silica gel packets that are in all closed compartments/drawers in the shop but they needed recharging (been in a year or so). I put them in the oven for 30 minutes at about 100C (212F) and have put a bunch back into the cabinet.
Ironically of all the tools like chisels, saws etc. that I have in a open tool rack exposed to the air of the shop, none of them have rusted at all! So much for a "protective" tool cabinet!
So just to underline - Please DO oil any steel tools or cast iron before putting them away.