Well I think it was the fingers; One of the things people forget sometimes is that every culture/country has it's own way of moving, gestures and mannerisms.
The way that people count on there fingers just one of the many bits of body language that is unique in every country.
Finger-counting varies between cultures and over time, and is studied by ethnomathematics. Cultural differences in counting are sometimes used as a shibboleth, particularly to distinguish nationalities in war time. These form a plot point in the film Inglourious Basterds, by Quentin Tarantino, and in the novel Pi in the Sky, by John D. Barrow.
A person indicating a numeral to another will hold up their fingers to signal the specific number. For example, a North American will raise their index, middle, and ring fingers vertically to signal the number 3.
For Continental Europeans, the thumb represents the first digit to be counted (number 1), as opposed to the index finger in North America. The index finger is number 2 through to the little finger as number 5. Fingers are generally extended while counting, beginning at the thumb and finishing at the little finger. For example, Europeans would use their thumb, and index, middle and ring fingers to express the number 4, whereas in North America they would use their index, middle, ring, and little finger.
Finger-counting systems in use in many regions of Asia allow the counting to 12 by using a single hand. The thumb acts as a pointer touching the three finger bones of each finger in turn, starting with the outermost bone of the little finger. One hand is used to count numbers up to 12. The other hand is used to display the number of completed base-12s. This continues until twelve dozen is reached, therefore 144 is counted.
When someone from North America or the UK count numbers on their hand it starts with the extension of the index finger (number 1) and continues to the little finger (number 4). The ‘number 5’ is then represented by extending all the fingers and thumb. The exact same process is repeated on the other hand if you want to count up to 10.
(i.e. Germans, Italians, Spanish, French)
When Western Europeans count numbers on their hands the thumb represents the ‘number 1’ and the index finger is ‘number 2’ and so on and so forth… with the little finger representing the ‘number 5’. Fingers are generally extended while counting, beginning at the thumb and finishing at the little finger.
So it is entirely possible that by using just one gesture he could have betrayed himself.