Why is it bad to bring propane tanks indoors?

I brought one into Walmart. And an employee said go around to the back. I don't understand why its dangerous have have a completely empty propane tank indoors. All the gas was used out of it. And I knew it had a safety release valve so if pressure builds up inside of it, it would release the contents without... show more I brought one into Walmart. And an employee said go around to the back. I don't understand why its dangerous have have a completely empty propane tank indoors. All the gas was used out of it. And I knew it had a safety release valve so if pressure builds up inside of it, it would release the contents without exploding. But there was NOTHING inside of it to build up pressure.
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