1 Samuel 31:6 states that all of Saul's men died:
So Saul and his three sons and his armor-bearer and all his men died together that same day.
So Ishbosheth surviving them would suggest he wasn't there.
Are there indications of why that might have been? Any examples of similar things happening with other families as a means of safeguarding succession or any allusions to Ishbosheth being too young/sickly/a poor fighter that I might have missed?