For some time I have noticed that the letterhead that gets attached to documents is location dependent, but it varies on the users logged in location NOT the location that the document was generated from

I understand that a client may visit 2 locations - 

For documents like invoices the letter head should always reflect the location the document was generated from the one that is associated with the invoice.  

For documents like statements - I would argue the location should reflect the customer.location not OpenVPMS.location as per the letterhead.

There are probably other documents this applies too.  

I can change our templates but I feel this is more an up the chain issue 

The use of the single letterhead template for all documents may not be entirely approproiate OR we need to consider the data being based to the template - perhaps rather than pass for example 

$P{dataSource}.getDataSource("customer")

We might pass the actual act to the invoice to allow the letterhead or pass the location.

IE $P{dataSource}.getDataSource(".")

or pass the location associated witht he act 

$P{dataSource}.getDataSource("location.target")

This is untested you might see that a customer that visits location 1...gets issued a invoice - and at the time the invoices shows location 1's details - a staff member latter resends the invoice while logged into location 2 - the person could think its a new invoice despite the invoice numbers being the same.  Its also not factually correct.