When planning a short trip to the States (with the intention to return in a week or two) it wouldn't really make sense to throw out a perfectly good toothbrush. Upon returning, Sun may have felt like she couldn't disrupt any of Jin's stuff. This sort of thing is very common among people who lose a spouse. This catch could tell us a little about the time line of events. Possibly, Sun has only been off the island for a short time when the flashforward takes place. If that is the case, she is not very far along in the grieving process and has therefore not come to grips with the reality of her life alone.