If the machine is owned and serviced by the third party, there may be no significant related activity in your customer's account. Ask your customer what the relationship is.
If the machine's owner is just renting space, ask for a copy of the lease. The payments to your customer may be nominal; e.g. convenience stores like to have ATMs on premise for the same reason they sell lottery tickets, to increase traffic.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.