I mean, yeah. But they'll probably notice when we start predicting their moves again. It took them several years to design a counter to our mind-reading last time, so just cos they know it's happening doesn't mean they'll be able to do anything about it.
I'm also thinking about the poor GM here. They need to be able to give little hints in the flavour text as to what the enemy is doing, I'm providing a way of doing so.
If we really wanted to keep our mind-reading secret, we'd have to do like the british in WWII and deliberately mess up frequently enough that the really important times where we get things right seem like coincidences. Which would limit the effectiveness somewhat.
I guess if you want to take a more subtle approach, we could exchange a bit of cost for secrecy;
Improved Detect Thoughts - Chain Readers: As some idiot suggests, we take advantage of the ease of casting by having up to a dozen mages working on a relay chain at once, each one handling the thoughts of one of the links. This would bump up the cost to Expensive, which would still be cheaper than Very Expensive. The Arstotzkans would be hard pressed to notice anything is amiss, as one of the things we would focus on in exchange for the increased cost would be secrecy.
Which we could combine with the following:
Order - Selective Intelligence Usage: We use IDT to gather all the intel we could ever want, but we only use the most important pieces- the ones that can geniunely swing a battle. This has the effect of making it seem like our increased success is mere luck, as opposed to magic (As in, we would hope that all that would be mentioned in the enemy's combat report is 'Moskurg has had better luck than we have', not 'Moskurg seems to know what we are planning')
To be clear, I am not voting for either of these. I'm just presenting the option. I still like the idea of granting Arstotzkans enlightenment as we steal their plans. Heh heh heh.