I heard that it is not acceptable for a husband to make a surprise visit to see his wife when, for example, one of them is traveling without the other and is not expecting the spouse to be there. I would like to know if this is true, and also does it apply in the other fashion (that the wife cannot make a surprise visit to her husband when he is not expecting her).

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
What you have heard about the reprehensibility of returning to one's house at night is correct. Imams Bukhari and Muslim reported from Jabir that the Prophet (Sallallahu Alaihi wa Sallam) said: "When anyone of you is away from his house for a long time, he should not return to his family at night" .
The statement of Hadith is specific for the one whose absence is for a long period and his wife does not know the time of his return; otherwise, the reprehensibility does not apply to the one whose absence is for a short period such as for one or two days, etc., or his wife knows when he will return. Hence, the reason for the reprehensibility, no longer exists in such condition. The general rule of Sharia is that the ruling (whether it is permission or prohibition) differs when the reason for the ruling changes.
The reason is the reprehensibility of returning at night is not to encounter his wife while she is not well-dressed and has not prepared herself to welcome her husband. This may cause hatred between them. Also, this reprehensibility is to keep one from mistrusting his wife and finding her shortcomings; otherwise, one may come at night intentionally for the purpose of spoiling the relation between him and his wife.
Basically, a husband is addressed by this reprehensibility but it applies to a wife too, since the reason is shared by them.
In addition, observance of the above rulings with persons other than married couples is also advisable, as mentioned by Imam Ibn Hajar in "Fatah al-Bari".
No doubt, the above ruling is a sign of the magnificence and eminence of this Sharia.
Allah knows best.