Looking for advice on planting the seeds for an NPC betraying the party

Keeping things purposefully vague in case any of my players are in this subreddit, but my party will soon be encountering one NPC in a small group of NPCs who will attempt to hire them for a quest (they’ve seen the posting and expressed interest, they will be meeting the NPCs next session). If they take the quest, the NPCs will accompany them on the quest.

In my ideal scenario, I’d be able to drop small hints along the way, but hopefully not so much as to blow the entire betrayal before we can get to the point of a climactic reveal in which she steals the MacGuffin they’ve been hired to help obtain.

The NPC who will betray them is a middle aged academic, reserved in a way that my players have found endearing in other NPCs. My goal is to get them to like her. Ideally I’d love for them to either develop a slight suspicion of her without fully knowing her plans, or be fully unaware but have everything click into place with the reveal. My players are smart, so I’ll still have ideas for what to do if they figure it out prematurely.

What are your tips and tricks for hinting at betrayal from an NPC before it happens, without revealing everything?