FYI, UA almost always does passports checks for International flights - particularly to Canada, where it's a legal requirement, but IIRC, this is company policy. In fact, only recently there was a thread where someone reported this didn't happen for them (believe different destination than Germany), but this is SOP for most US carriers, exactly to make sure you have it so they don't get fined. International carriers don't always do this, for example, almost al my Ex-India flights (SQ is the exception, IIRC), they don't check passports at the gate - presumably since you need it to do exit immigration anyway. UA is the other exception. Going back to India, they do check for passport and visa.

The difference with Germany, IIRC, compared to other countries, is on their insistence on a signed copy of your passport. They are apparently really strict on this. I found out at check in here at CVG, with my daughters first flight when she was 3 months old. I didn't sign the passport at the time we got it, because I didn't know how that worked for a minor. The agent said I needed to sign it as the parent, and despite not even flying to Germany (we were flying to YYZ), agent specifically cited Germany as a place where we wouldn't be allowed in if it was t signed beforehand. And this was with outsourced staff at an outstation, not even a hub with flights to Germany.