Please read through all of the replies, there were several suggestions to determine if the expansion tank has failed.

It’s Southern thing and well water thing. Only time I seen expansion takes used was on well water until I got down here.

Expansion tanks are required by almost all plumbing codes where the water system is a closed system with a backflow preventer. It's pretty much a universal requirement. Not just in the South.

Expansion tanks are unnecessary and the reasons for them are silly code or not.

Not where I’ve been.

1985
 

Around 1985 Municipalities (water companies) started installing water meters with back flow preventers in new homes. This was done to prevent contamination of the Municipal water supply. Doing this created a closed system for each home with this type of meter. Prior to this the excess pressure would not build in your home, the excess pressure would go into the entire Municipal system. Now it can't get past the water meter. So now it builds in your home. Then the water is turned on and the pressure is relieved. Water off pressure builds again. Plumbing systems are built to withstand this. So what's the big deal? The problem is not with the pipes in the system, but with the weak points, the hoses and plastic tubing. ie. washing machine, ice maker supply line, toilet supply. Constant expansion and contraction will weaken them until they burst. This is why I believe in expansion tanks.

About 12 years ago, I had an old water heater replaced in Virginia without an expansion tank. Within a few days, I noticed a small amount of water in the drain pan. It was because the new relief valve was occasionally tripping and releasing water. That means that the water pressure was exceeding 150 psi. The contractor said that the relief valve on the old water heater was totally clogged with sediment, so it did not function at all. The contractor installed an expansion tank, and I never had the problem again.