Moisture Trapped in IP66 Enclosure
Here's what happened to a product I designed that was IP69K (Protection against powerful high temperature water jets). It was installed in China and failed after about a couple of years. When looked into there was about 1.5 inches of water slopping about. Seemingly a mystery.

However, it turned out to be a progressive build up of humidity. Every night the product was turned-off and internally it started to cool over night. During that period the internal pressure lowered (because the internal air cooled) and it sucked-in high-humidity air from the surroundings. That settled out on top of the microscopic layer of water that was ingested the previous night. Gradually it built up until it reached a switch mode power supply then bang.

The way we got out of this was selling the option of having a drain plug fitted and each order was processed with attention to that humidity problem.

Silica gel will work but only has a limited lifespan before it needs to be replaced. A positive pressure inside will help - if it's sealed as good as you suggest you can buy slow leak gas cartridges that keep the pressure positive - they are used in the petrochem industry and are akin to flameproofing by using pressure to stop volatile gases entering and potentially being ignited.