Saying it's sneaky, sounds like you feel you have a right to know what's going on in her life, which might be a clue as to why she didn't tell you.
But there are plenty of quieter reasons which people have said here. A surprise, not wanting to jinx it, not sure it was going through etc.
I suspect dh's brother might have said something similar. He was round at ours telling us how he was buying a house, and being very condescending that he thought we wouldn't be doing that for ages because we weren't as well off as him... wasn't that such a pity?
We didn't say anything. Not only because getting a word in edgeways with him is challenging and he doesn't listen anyway, but also he would have then spent the time telling us that his was better than ours.* But also because we had some money coming to help us pay for the deposit, but the person involved had asked us not to tell anyone until we'd finalised the arrangements.
*and in case you don't believe that, he told my mil when he knew that his house was far bigger than ours when he knew nothing about our house except it was 3 bed, like his. This did lead to a rather funny moment when we went round and he was showing us round, and dh asked if he could see the rest of the house too... 