If a 2010 Highlander's alarm keeps going off, what is the problem?

It may sound odd, but if Your 2010 Toyota Highlander's anti-theft alarm keeps going off, it's very likely that Your hood latch is to blame. Toyota issued technical service bulletin T-SB-0200-10, which states that certain model years of the Highlander (2008, 2009, 2010, and 2011) are known for a defective hood switch. If Your VIN begins with 5TD, this is likely Your issue. The issue also persists on Highlander Hybrid models having a VIN beginning with JTE. Overall, it's a relatively simple fix -- if Your vehicle was under warranty, it was covered by Toyota. As this is likely no longer the case for a 2010 model year, You're looking at around $100 for a replacement, and it should take no more than an hour of Your time.