Absolutely agree on recon being vital to sandbox games, and its a key component to controlling pace as well. I've set up the campaign I'm currently running in what I think of as Metal Gear Solid 5 style. The players have a large area to explore, and just by walking around, may run into a combat or two. They will also run into story hooks.
Once they take one of those hooks, they are in a mission. That mission will have between 10 and 15 combat encounters possible in it. Through recon, the players may learn ways around some of those encounters. Through skill checks, they may get around others. Their goal is to reduce the number of encounters they will face to a number low enough to accomplish their goal, typically between 6 and 8 (including multi-part encounters) since abandoning their goal to take a long rest will almost always lead to either failure to accomplish the goal, or at the very least when they return will find the difficulty went up. Short rests also risk upping future encounters as the enemy has time to regroup and prepare, but I will provide hints through the recon or skill checks as to when and where the players may find opportunities to short rest without that concern.
Along the way, I'll tempt them with encounters that lead to treasure but don't directly affect their goal. If they've successfully avoided other encounters, they may have the resources to go for it, if not, they may have to decide whether to give up their primary objective.
If they are attempting to do something "beyond their level" and they did not discover that in recon (or ignore what they discover), I telegraph early into starting the mission proper just how tough the situation is by throwing a deadly fight at them. They should win that fight, but also should be very nervous about pushing on further until they have acquired more power, which leads to another story hook as they go looking for the magic sword or staff of power that can help them accomplish that goal.
If all that sounds like a lot of work, well, running a sandbox is IMO! That said, I'll typically have a "patrol" type encounter for each mission that can be reused 4-6 times either on their own or as the back of a multi-part encounter if the players let someone in the first group sound an alarm. Several other encounters may be duplicates that are in different areas, and are only triggered with unlucky skill check rolls to avoid.