Here is the difinitive answer. My answer is based on over 10 years experience and first hand knowledge as I work in the insurance industry.
Your equipment is covered under your homeowners or renters policy. It is covered both while at your residence and away from your residence. Coverage is provided regardless of the amount of time that it spends away from your residence. The key to coverage is ownership not location of the item owned. It is covered if stolen out of your car under the homeowners or renters policy and NOT your auto policy. A deductible WILL apply. You are responsible for choosing the amount of the deductible. If you leave your rig at someone elses house it is NOT covered under their homeowners or renters policy, it is covered under your policy. There may be a restriction on coverage outside of the United States but this varies by carrier. Musical equipment does not have an automatic limitation of coverage like jewelery does.
Coverage also depends upon the cause of the loss. Policies are written either as a named peril (theft, water, lightning) or all risk with excluded causes. Therefore spilling a beer on your bass head might or might not be covered depending upon your policy.
If something is stolen file a police report right then and there. Do NOT wait until the next day and do not file a bench report. The police report is the FIRST piece of documentation that an insurance company looks for and often there is a condition in the policy that states a police report must be filed.
The next thing to remember is documentation, documentation, documentation. By this I mean original documentation/verification of purchase. Do not expect your insurance company to accept a written estimate of value written by a shop after a loss has occurred. Next are photos of the equipment especially the serial number.
Finally, most policies are written on a replacement cost basis. So if you score a really great deal like finding a early 70's Fender Tele bass at a garage sale for $25, your insurance company will owe you the market or replacement cost of that bass, not the $25 you paid for it. If you have vintage equipment, get a written appraisal.
If you have any specific questions about insurance, carriers or need help in the unfortunate event that you have a loss PM me.