The explanation I can only surmise.

The Praguer is an exceedingly acute person; some subtle falsity of accent, some slight grammatical inaccuracy, may have crept into our German, revealing to him the fact that, in spite of all appearances to the contrary, we were no true-born Deutscher.

I do not assert this; I put it forward as a possibility.
To avoid unnecessary danger, however, we did our sight-seeing with the aid of a guide.

No guide I have ever come across is perfect.

This one had two distinct failings.