[bsvalley]

Interviewing requires a lot of experience and you have to be extremely humble. You may fail at capturing excellent candidates in only 30 minutes based off your own judgement. The more curious and patient you are, the better. In the scenario you've described, flagging the candidate means you didn't get the exact answer you were expecting after you asked a question, which is a terrible judgement. The red flag should be on the interviewer side.

Your role as an interviewer is to extract the information out of a candidate, otherwise we'd be using programs to do that for us because it's not just a 'yes' or 'no' answer. What I would do if I were you is to ask a follow up question - "I'm actually interested in learning more about that deep learning piece you mentioned, could you elaborate more on that, for example, is it using xyz technique or abc?". If the candidate can't answer your follow up question then follow up with "do you have experience in deep learning?". Only then, you'll get an accurate answer to your question. There's no guessing nor red flagging. If it's a yes then dig more until you get to understand the level of exposure to deep learning for this candidate. As you can tell, it involves 2 things from you: 1. be on top of deep learning if it's a requirement for this role, 2. being curious and ready to triple check your judgement. It's not a "I know, you don't know" type of situations, it's about trying to understand the level of a candidate within certain topics, while pushing the conversion in the right direction. That is your role as an interviewer.

Last but not least, it's totally fine if the answer you get is "I don't know anything about this topic". Try to find out if the candidate would be willing to learn deep learning and the reason why he/she would be interested in learning that. You may not have a deep learning expert in front of you, but maybe this person has the potential to become the next DL guru based on other things that could highlight excellent problem solving. You have to bridge the gaps as an interviewer by staying positive until the end - I'm sure this candidate has something to offer, let's try to find out what is that super power!