When using the Custom item Send To Location feature of a Document Library, the item's Last Modified datestamp gets updated in the source location. As I like to start a workflow on this event, could anyone tell me what field has changed that triggers an item modification in this case? I tried turning on version history on this Document Library, however no new version is created - so I am confused.
In the Send To option, there are 4 options -  Other location,  Email a Link,  Create Document Workspace and  Download a Copy. I believe that you have select  Other location.
From my testing, I notice that there are no changes to the item's Last Modified date-stamp for the Document library that you select 'Send To Other location'. I also tried the 'Send to Download a Copy' and the date-stamp did not changed at all. The only changes is that the date-stamp of the document in the new location, which reflect the date & time that you execute the 'Send To Other Location' operation.
I believe that if you have made changes to the document on the current document Library, it will update the version history. Since you use the 'Send To Other Location', which basically just copy the document from the current document library to another document library, the version history will not reflect this operation.
Probably, you can let us know what are the steps that you have taken so that I can do the testing at my side and will be able to give me a better answer.
Hope it help.