What is a realistic scenario where the Axis discover the allies have breached their most sensitive secrets?
I think the consequences would generate many butterflies, but not enough to change the outcome.
And there would be consequences. Any nation would try to figure out how and why their secrets aren't so secret. I'd expect code changes, but also plenty of arrests and purges.
otl possible moments figuring out that enigma was compromised:
1. capture of one general or another who spills the beans or has compromising documents around
2. the germans read deeper into their success in the channel dash; something along the lines of hey, we kept this pretty secret and only communicated on the local phone network and caught the british totally flat footed I wonder if we should try that again
3. reading deeper (a la above) when rommel advanced against enigma orders such as the first cyranaica offensive or the rebound from crusader and caught the british flat footed
4. rommel not dashing to the wire after whipping the 8th army at sidi rezegh during operation crusader and instead massing both panzer divisions and the xx italian corps to encircle and finish off the british xxx corps; if their war diary was captured (forgetting over informed people) a strong investigation of it would show that german mail was being read
major pod but plausible way for the germans to find out
something akin to my operation full moon in manstein in africa; namely a parallel german and italian operation; where the german component gets crushed by prepositioned forces and the italians have a cake walk
italian military codes were unbreakable (they used one time cipher pads of the sort that were very popular with the kgb in the 70's and 80's... tom clancy's red rabbit has an excellent description of how they work)... the british couldn't read italian military signals until the germans forced them to switch to enigma in 1942
so some sort of investigation of a battle like full moon would find the difference in signal equipment as a possible reason for the disproportionate results