Those early April encounters between the armies did nothing to convince Grant and Sherman that an attack was eminent. Some Union commanders, usually at the regimental level, did think that something was going to happen. Colonel Peabody was one such leader and as a brigade commander was probably the highest ranking officer to believe that something was wrong. On the night of April 5 he was having a hard time sleeping because he kept thinking about the unknown danger so he decided late that night to send out a reconnaissance party. The party originally was about 3 companies strong but he later sent more men out there. The orders were simply to keep going until they found out what was out there. Around 5 A.M. on April 6 they fired the first shots of the battle.
Peabody did not have to send out the expedition, in fact he was violating orders when he did so. Prentiss had told him during the day not to send out an expedition or to even get his brigade ready for a morning attack.
Peabody's men held the Confederate advance back about an hour which allowed Prentiss' other brigade to be alerted. If there had been no warning the initial Confederate attack on Prentiss' division may have been more catastrophic. Also the Confederates might have been able to get troops everywhere along the front before the warning was raised. This might have meant a quicker fall to Stuart’s camp and allowed the Confederates to gain the Peach Orchard before Hurlbut’s division came up.
Verdict: Peabody provided the first warning of battle. Eventually someone else would have raised the warning but the Confederacy might have been in a better position at that time. A more unified opening attack would have been a great advantage for the Confederates so this qualifies as a critical decision.