The accent makes it difficult to determine what's being said without having the actual voice actor on hand. However, using Google we can determine what the correct phrase is (and therefore, what he most-likely meant to say).
Google gives the following estimated result-sizes when searching for these phrases (in quotes):
- sneaky peeky: 85,000
- sneaky peaky: 50,900
- sneaky beeky: 12,200
- sneaky deaky: 5,900
- sneaky deeky: 1,090
- sneaky beaky: 463
Additionally, google n-gram shows that only one of these phrases is used in real literature (or at least, the literature they have indexed):
Therefore, it's easy to conclude that the correct phrase is "sneaky peeky".
[Edit April 2015] People have pointed out that there's now a CS:GO sticker with the phrase "sneaky beaky" on it.
According to the author of that sticker, it's supposed to be a joke:
Haha, it's "sneaky peaky/peeky".
This is just a play on words, because a) it's really hard to tell what the hell they're saying in game; b) chickens are a huge part of cs and c) chickens are sneaky, and they have beaks... sneaky beaky. Yeeea.
However, because of that sticker, "sneaky beaky" appears to be becoming an 'acceptable' alternative to "sneaky peeky". Google's estimated result size for "sneaky beaky" has gone up from 463 (Feb 2014) to 101,000 (April 2015)!