PA-44 Seminole/Turbo Seminole
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 9.7
Situational Awareness
Airspace Violation All Types; Conflict Airborne Conflict
Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Airspace Structure; Human Factors
I had just relieved Groung Control/Controller in Charge and Local Control  was either giving a relief briefing; or about to start the process.  The outgoing Local Control Controller observed a 1200 code outside the Class D airspace and transmitted in the blind to see if the other aircraft was on frequency.  He was able to raise the aircraft; which I believe was Aircraft X.  The pilot advised they were trying to shoot the gap between the Class D and Class B airspace.Aircraft Y was inbound to BJC for a visual approach; and appeared to be overtaking the slower VFR traffic ahead.  Local Control called traffic and appeared to issue control instructions to try to resolve the conflict; as well as coordinating with the approach controller.However; when I looked through binoculars to see the positions of the two aircraft; I observed the inbound Aircraft Y in a bank out towards the east; and it appeared they were taking evasive action.  I advised the controllers at Local Control of my observation.  After the maneuver; Aircraft Y continued inbound for a normal landing.Another occurrence in the 'corridor' east of BJC Class D airspace where a VFR aircraft transitioning on its own conflicts with high performance inbound/outbound traffic.  As traffic continues to increase at BJC and in the Denver area; additional actions should be taken to encourage VFR traffic to either receive radar advisories or stay away from the arrival/departure paths used by high performance aircraft at BJC and near the Denver Class B.  Even if there is no way to prevent VFR aircraft from entering this 'corridor' on their own; encouraging voluntary compliance from pilots may still increase safety for all involved; while reducing controller workload.
BJC Tower Controller reported an aircraft making evasive actions due to a VFR aircraft trying to shoot the gap between the Class D and Class B airspace.