The bit you are trying to force is definitely not a real, physical I/O bit.
All Logix5000 I/O tags are created automatically when an I/O module is inserted into the I/O Configuration.
All Logix5000 I/O tags have the generic format of :-
Location : Slot : Data Type
Location - will always be "Local" for modules in the same chassis as the controller. When located in a remote chassis, the Location is the name assigned to the remote communications module in the I/O configuration.
Slot - speaks for itself, slot number where the module resides in the chassis (remember they are numbered left-to-right, starting at Slot 0).
Data Type - This will be either I, for data coming Into the controller, or O, for data going Out of the controller.
[Note : There will be another tag created associated with a module, that is the "C" tag. This tag contains the module configuration which is downloaded to the module at power-on, once accepted, the module will communicate with the controller.]
Typically then, I/O tags will look like Local:4:I and you open up these tags to get at the I/O data.
Examples :-
You want to look at bit 7 of an Input module in slot 14 - the tag will be Local:14:I.Data.7.
Output bit 26 of an output module in slot 9 of a remote chassis via an Ethernet or ControlNet communication module (doesn't matter which) named "Albert" in the I/O configuration will have the tag Albert:9:O.Data.26
These I/O tag formats are fixed, therefore you I_3.11 is not an I/O tag. Because it is named that way (i.e. I_3), I would guess that the system has been converted and updated from an earlier hardware platform (e.g. PLC5 or SLC) that uses I/O addresses.
As has already been said, it is most likely this tag is "mapped" from the real I/O point somewhere in the code, and cross-referencing it will reveal where.