That's normal.
1. If NPCs see you stealing, they will catch you.
2. If NPCs within the area they are patrolling suddenly notice that objects that were there before are missing, they will proactively look for suspects.
Thank you for responding. If my party moves far away from the missing items is there a timer on how long an npc will look for suspects or is it coded that she'll notice the party passing by as the thieves even after, say, days of not being around the npc?
The timer is over when the NPC stops patrolling and returns to their original post. As there are, however, sometimes multiple available patrolling routes, there's no rule for how long it will take.
Remember to send your items to camp immediately after you stole them. Don't leave any PCs near any NPCs if possible. There are "guards" which will catch you, or somehow just know without walking up to you if you do. This was particularly evident in Druid's grove as early as EA.
They should also notice if you wear stolen items. It's safer to buy gear with gold, then steal the gold. Too much gold at once makes passing the check near impossible.