I would back out the price impact of factors that are probably beyond the control of the traders. For example, in corporate bonds, I would remove the price impact of the local interest rate moves and the impact of any currency effects. The credit spread impact is a little difficult in that the insider information would probably also impact the credit spread. For this factor, one could take the approach above where a 2 sigma move on the credit spread could alert one to a suspicious trade. This is an example for a credit risky bond. Of course if one is to look at information leakage from central banks etc, one would need to look at factors that impact those bonds for large moves, and local interest rates would of course need to be included in that analysis. Also, one could look at similar instruments to see if an expected impact on those instruments were in sync with the move in those bonds. For example, did the equity also get impacted similarly (taking into account the subordination)? What about the options, asset swaps, credit default swaps, etc.?
I would also look at volume spikes.